Thermoplastic substrates ordinarily are based on converting thermoplastic resins, powders, granules, pellets and similar forms of thermoplastic resinous material under heat and pressure to form useful molded articles. Injection molding processes for molding thermoplastic resinous materials typically comprise heating the molding compound to form a viscous flowable melt, injecting the heated melt under high pressure into a relatively cool closed mold cavity, cooling the melt to form a solid shape molded substrate conforming to the interior configuration of the mold cavity, and then ejecting the molded part from the mold cavity. Ordinarily, the compounded molding resin flows under pressure into a heated mold to conform to the shape of the mold cavity. The mold is closed and the heat and pressure is maintained for sufficient dwell time to fully form the intended plastic article. The mold and formed plastic article are cooled to harden the molded plastic article, whereupon the mold can be opened and the molded article removed. In thermoplastic molding processes, the molded part can be a finished article having a wide variety of design details such as bosses, flanges, ribs, bushings, holes or other openings, various functional structures, decorative designs, and flat surfaces. Most molded substrates need to be surface coated, particularly for appearance purposes, and require good adhesion to the applied surface coating. However, many desirable decorative and/or protective surface coatings are difficult to adhere directly to thermoplastic substrates, especially to nylon and modified nylon substrates.
In-mold coatings typically are sprayed into the mold cavity to coat the interior mold surfaces during the molding process to provide a surface coating integrally fused or adhered to the thermoplastic substrate being injection molded. In-mold coatings have been found to be particularly advantageous for molded substrates to provide a useful surface treatment to the molded substrates. In-mold coatings avoid surface imperfections in the molded part such as surface porosity, sink marks, surface waviness, and similar surface defects and imperfections, which frequently require additional labor and costs to rework and post finish. An in-mold coating can be injected into a slightly opened mold or injected under pressure into a closed mold. The in-mold coating is applied to the mold cavity surfaces and/or applied over a molded or partially molded substrate, and then heat cured in the mold under pressure to form a thermoset surface coating integrally fused on the molded substrate. An in-mold coating can be injected into the mold after the mold pressure is released and while the mold is opened infinitesimally to permit injection of the in-mold coating into the mold cavity. For instance, U.S. Pat. No. 5,902,534 discloses a method of injecting molding resin into a mold, followed by injecting an in-mold coating into the mold cavity between the molded substrate and the mold cavity surfaces, compressing and curing the in-mold coating while in contact with the mold cavity surface, and then cooling the coated molded substrate to provide interface adherence of the cured in-mold coating to the molded substrate surface. Similarly, U.S. Pat. No. 4,668,460 suggests a method of in-mold coating a molded substrate contained within a closed mold by first molding the substrate under pressure and then injecting an in-mold coating into the closed mold at a pressure higher than the molding pressure. In-mold coatings are primarily directed to producing smooth surfaces free of surface defects and imperfections.
In-mold polymeric urethane-acrylate coatings containing copolymerizable acrylates and/or ethylenically unsaturated monomers are disclosed in U.S. Pat. No. 4,189,517, No. 4,222,929, No. 4,331,735, No. 4,414,173, No. 4,508,785, No. 5,084,353, No. 5,132,052, No. 5,658,672, and No. 5,777,053.
It now has been found that an in-mold appearance surface coating comprising an urethane acrylate oligomer adapted to addition copolymerize with other ethylenically unsaturated components, including minor amounts of a sulfonic acid functional acrylate monomer and an acrylic acid monomer selected from acrylic, methacrylic or ethacrylic acids, provides an improved in-mold surface coating with excellent adhesion to difficult adhesion substrates, such as thermoplastic nylon and modified thermoplastic nylon. The cured in-mold surface coating produces a finished appearance free of surface defects and imperfections and avoids costly refinishing or reworking of the molded parts. These and other advantages of this invention will become more apparent by referring to the detailed description of the invention and the illustrative examples herein.